1. Field of the Invention
The present invention generally relates to an organic light-emitting device (also known as OLED) and a method for manufacturing the same and, more particularly, to a white light emitting organic electroluminescent (EL) device able to directly cast continuous full color light containing three different frequency bands and a method for manufacturing such an organic electroluminescent device.
2. Description of the Prior Art
The organic electroluminescent device, upon which C. W. Tang and S. A. Van Slyke (Eastman Kodak Company, Rochester, N.Y.) have made efforts since 1987 so as to deposit hole/electron transmission materials such as aluminum trisoxine [a.k.a., tris (8-quinolinol) aluminum] by vacuum evaporation on an indium-tin oxide (ITO) glass followed by the deposition of a metal electrode, has attracted tremendous attention due to its advantages over other display panels. These advantages include self-luminescence, large visual angle, short response time, compact size, light weight, reduced dimension in thickness, high brightness, low power consumption, simple fabrication, and the ability for light emitting in a full color range. Therefore, such an organic electroluminescent device is increasingly required to replace the currently used white light sources such as fluorescent lamps and light bulbs so as to save energy, and the technologies thereon have widely been studied in the industry all over the world.
Please refer to FIG. 1, which is a cross-sectional view showing the structure of an organic EL device disclosed in U.S. Pat. No. 4,769,292, issued Sep. 6, 1988, filed Oct. 14, 1987 by Van Slyke et al (Eastman Kodak Company, Rochester, N.Y.), entitled “Electroluminescent device with modified thin film luminescent zone.” The organic EL device 10 comprises in sequence: a transparent substrate 11, a light transmission anode 13 formed of tin oxide or indium-tin oxide (ITO) by evaporation, a hole transmission layer (HTL) 15, a luminescent layer 17, and a metal cathode 19. The luminescent layer 17 is formed by a thin film comprised of an organic host material capable of sustaining hole and electron injection and a fluorescent material (not shown) capable of emitting light in response to electron-hole recombination. When an external voltage across the anode 13 and the cathode 19 is applied to the device 10, the anode 13 injects holes (positive charge carriers) through the hole transmission layer 15 into the luminescent layer 17 while the cathode 19 injects electrons through the hole transmission layer 15 into the luminescent layer 17. The portion of the luminescent layer 17 adjacent the anode 13 thus forms a hole injecting and transporting zone 15. The injected holes and electrons each migrate toward the oppositely charged electrode. This results in electron-hole recombination to form an exciton within the organic luminescent medium 17, which leads to energy released as light according to the chosen fluorescent material.
The afore-mentioned prior art organic EL device has advantages in good quality and enduring lifetime. However, the structure employed can only cast monochromatic lights according to various chosen fluorescent materials, and fail to achieve the objects of emitting white light or continuous full color light.
Please refer to FIG. 2, which is a schematic band diagram showing the structure of an organic EL device disclosed in U.S. Pat. No. 5,668,438, issued Sep. 16, 1997, filed Jun. 6, 1996 and U.S. Pat. No. 5,886,464, issued Mar. 23, 1999, filed Apr. 18, 1997 by Shi et al (Motorola, Inc., Schaumburg, Ill.), both entitled “Organic electroluminescent device with emission from hole transmission layer.” In the EL structure, an anode 22 is formed of tin oxide or indium tin oxide (ITO), an organic hole transmission layer (HTL) 23 is formed on the anode 22, an organic electron transmission layer (ETL) 24 is formed on the hole transmission layer 23, and a cathode 25 is formed on the electron transmission layer 24. The materials for the hole and electron transmission layers 23 and 24 are so selected as to satisfy the following inequality:
 (EC1−EC2)<(EV1−EV2)
where EC1 and EV1 respectively represent a conduction band level and a valence band level of the material selected for the hole transmission layer 23; and EC2 and EV2 respectively represent a conduction band level and a valence band level of the material selected for the electron transmission layer 24.
The inequality ensures that the energy barrier for holes to be injected into the valence band of electron transmission layer 24 from the valence band of hole transmission layer 23 is greater than that for electrons to be injected into the conduction band of the hole transmission layer 23 from the conduction band of electron transmission layer 24. In other words, the number of electrons to be injected from the electron transmission layer 24 into the hole transmission layer 23 is much larger than the number of holes to be injected from the hole transmission layer 23 into the electron transmission layer 24. Therefore, electrons and holes recombine in the part of hole transmission layer 23 close to the interface of electron transmission layer 24 and hole transmission layer 13, where light emission occurs. Moreover, in order to facilitate holes to be injected into the hole transmission layer 23 from the anode 22, the EL structure further provides a hole injection layer interposed between the anode 22 and the hole transmission layer 23.
Although the afore-mentioned prior art organic EL device has high electroluminescence efficiency due to light emission from the hole transmission layer 23. However, the structure employed can only cast monochromatic lights according to various chosen fluorescent materials, and fail to achieve the objects of emitting white light or continuous full color light.
In recent years, there are several methods that have been investigated and developed by the industry to realize an organic EL device capable of emitting white light or full color light, including:
1. Color conversion: In this method, a monochromatic light passes through a color conversion material composed of different color conversion layers and is then resolved and converted into light with different colors, e.g. three primary colors, such as red, blue, and green so that an organic EL device capable of emitting white light or full color light can be obtained. However, this method also suffers from a number of problems. First, for example, most of the available color conversion materials are not satisfactory in color purity and luminescence efficiency. Secondly, the background light (such as blue light and UV light) may also be absorbed by the color conversion layers, which often leads to poor contrast and defective pixel quality.
2. Color filter: In this method, white light is used as the back-lighting source of the organic EL material. It is useful to achieve full color light when accompanied by LCD color filters. However, the key problem of this method is how to obtain a reliable white light.
3. Three independent colors (RBG): In this method, three primary colors red (R), green (G) and blue (B) are independently demonstrated to realize a full color display or a white light source. However, since the three colors are independently demonstrated, RBG pixels require different driving voltages. It suffers from complicated fabrication process and larger size. In addition, in such a device, high precision is critically required for the RBG pixels. It is found that the fabrication process may be difficult and complicated. On one hand, the RBG pixels formed of three different organic EL materials may have different luminescence efficiencies, lifetimes, driving conditions. For example, the red light shows poor purity and may shift to orange color. The red light also has shorter lifetime and may adversely affect the overall performance of the display. On the other hand, the method is performed by a two-wavelength approach; therefore, chromatic aberration may occur.
Therefore, the present invention has been made to solve such problems in view of the forgoing status and to further provide a method for manufacturing a white light emitting organic electroluminescent (EL) device able to directly cast continuous full color light containing three different frequency bands so as to realize white light emitting.